Hackers have managed to circumvent the BBC's anti-piracy systems to gain unrestricted access to the programmes on the corporation's iPlayer internet TV service.

The loophole potentially allows millions of people to download television shows on to their computers without any copyright protection.

The iPlayer system, which lets surfers watch BBC videos online in a similar way to websites such as YouTube, has proved massively popular since it launched late last year. Shows such as Top Gear and Doctor Who can be watched over the web or downloaded to a computer for viewing at a later point.

When developing the iPlayer, however, the BBC agreed to include anti-piracy measures in all programmes in order to appease TV production companies and broadcasting unions. The addition of so-called digital rights management (DRM) software means new shows disappear from the website after one week, and files downloaded on to a computer have a 30-day expiry date. After the limit is reached, the videos are no longer viewable.

However, the backdoor in the iPlayer allows users to completely bypass those protections and download unprotected versions of any show on the website.

One of the hackers responsible said it took him only 12 minutes to find the loophole and make it work - but that such a simple hack potentially opened up every programme on iPlayer for unlimited downloading.

"The BBC accidentally opened the floodgates and gave the world DRM-free downloads," said one hacker with knowledge of the breach. "If only it were down to something other than poor design, decisions and ineptitude."

The BBC said the system was still in its trial phase, known as "beta", and that it was already working to come up with a solution.

"This is not unusual or surprising," said a BBC spokeswoman. "We have made it clear the BBC iPlayer on iPhone and iTouch is currently in beta, which enables us to pick up on such issues and find a solution before we roll the service out in full in due course."

The iPlayer was launched last year at a cost of several million pounds, and initial reaction to the download service was modest. But the ability to watch online - which was added last Christmas - has proved highly successful: the most recent figures indicate that programmes were viewed more than 17m times between December 25 and mid-February.

However, the decision to incorporate anti-piracy protections has angered some campaigners, who believe that British citizens should have unprotected access to shows for which they have already paid with their TV licence fee.